Protozoa are unicellular eucaryotic microorganisms that lack cell walls. They are generally colorless and motile. Protozoa are distinguished from bacteria by their usually greater size and eucaryotic nature, from algae by their lack of chlorophyll, from yeast and other fungi by their motility and absence of a cell wall, and from the slime molds by their lack of fruiting body formation.
Protozoa usually obtain food by ingesting other organisms or organic particles. Protozoa are found in a variety of freshwater and marine habitats. A large number are parasitic in other animals, including humans. Some protozoa are found growing in soil or in aerial habitats, such as on the surface of trees.
As is appropriate for organisms that "catch" their own food, most protozoa are motile. Their mechanisms of motility are key characteristics used to divide them into major taxonomic groups. Protozoa that move by amoeboid motion are called Sarcodina (common name: amoebas); those using flagella, the Mastigophora (flagellates); and those using cilia, the Ciliophora (ciliates). The Sporozoa, a fourth group, are generally nonmotile and are all parasitic for higher animals.
Typical representatives of the Mastigophora group are Trypanosoma (causing African sleeping sickness), Gtardia, Leishmania, Trichomonas and Tritrichomonas. Members of this protozoal group are motile by the action of flagella.
Among the sarcodines are organisms such as Amoeba and Entamoeba which are always naked in the vegetative phase. Entamoeba histolytica is a good example of these parasitic forms.
Ciliates are defined as those protozoa that, in some stage of their life cycle, possess cilia. Typical representatives are Balantidium and Parameclum. Balantidium coli is primarily a parasite of domestic animals, but occasionally it infects the intestinal tract of humans, producing intestinal symptoms similar to those caused by Entamoeba histolytica.
The Sporozoa comprise a large group of protozoa, all of which are obligate parasites. Typical sporozoans are Plasmodium (causing malaria), Toxoplasma and Cryptosporidium.
Habitats of endoparasites vary. some are intracellular, such as malarial parasites in vertebrates. Other parasites, such as, Entamoeba histolytica invade tissues but not individual cells. Most trypanosomes live in the blood of vertebrate hosts. Many other parasites live in the lumen of the digestive tract or sometimes in coelomic cavities of invertebrates, as do certain gregarines.
Free-living and parasitic protozoans are the cause of many diseases and infections in humans and animals. For example, the protozoan parasite, Entamoeba histolytica, is recognized as having infected almost 480 million people. There is no known animal host for the parasite. While the major part of E. histolytica infections are of a non-pathogenic nature, over 36 million cases result in clinical disease.
The major proportion of clinical cases of E. histolytica infection are present in the form of amoebic dysentery. The amoeba form ulcers in the large intestine. This causes diarrhoea and bleeding into the lumen.
Clinical manifestations of E. histolytica infection are mostly restricted to the large bowel. However, if untreated, some of these infections can result in perforation of the large intestine and, under extreme conditions, death from peritonitis.
In other cases of E. histolytica infections, extra-intestinal invasion of the parasite may occur. The parasite passes, via the blood or lymphatic system, to abdominal and other organs to produce amoebic abscesses. Under extreme conditions, these infections can also result in severe disease or death.
Metronidazole has been the drug of choice for the treatment of E. histolytica since the first report on efficacy was published in 1966 (Powel et al., Lancet 2, 1329-31 (1966)). The acute diarrhoea and the liver abscesses associated with E. histolytica infection respond well to metronidazole. However, metronidazole has been found to be less effective in asymptomatic cases (Spillman et al., Am. J. Troy. Med. Hvg. 25, 549-51 (1976)).
Failure of treatment of E. histolvtica infections with metronidazole has been reported with international flight personnel in individual patients (Weber et al., Trans. RQV. Soc. Troy. Med. Hvg. 84, 803-5 (1990)). A randomized study of fifty South African patients with amoebiasis revealed a treatment failure with persistent disease in 55% following standard metronidazole therapy of 2.4 g daily for 5 days (Jackson et al., Symposium on Amoebiasis, New Delhi, India (1990)).
Although metronidazole is effective in treating E. histolytica infections, it has several undesirable side effects. For example, it cannot be administered to pregnant women who are in their first trimester. Moreover, the utility of metronidazole is seriously compromised by its interference with alcohol metabolism. The drug is mutagenic in the Ame's test (Vogd et al., Mutat. Res. 26, 483-90 (1974)). Because of its toxic side effects, metronidazole has been covalently linked to silica beads to abolish drug absorption in the treatment of intestinal amoebiasis. Although short term use of metronidazole has been relatively safe, toxic effects of testicular atrophy, ataxia and polyneuropathy has been observed in rats. Metallic taste, headache and peripheral neuropathy are also observed.
Due to the problems associated with the use of metronidazole, there is a need for an alternative method of treating infections by protozoan.
The antibiotic bacitracin has shown a limited capacity to kill both pathogenic and non-pathogenic forms of intestinal amoebic infections (Most et al., Am. J. Troy. Med. 30, 491-497 (1950)). For example, in 51 patients, a parasitological cure was obtained in 66% of cases following treatment for 10-20 days with dose ranges of 40,000 to 160,000 units. Extension of therapy beyond 10 days, or an increase in dose to about 80,000 units, did not significantly enhance the probability of cure beyond 66%. The marginal effectiveness of bacitracin in treating amoebiasis has not led to further use of this drug in the treatment of E. histolytica amoebiasis.
Another common disease caused by protozoans is bovine trichomoniasis. Bovine trichomoniasis is a sexually transmitted disease caused by the flagellate protozoan Tritrichomonas foetus. Infection by T. foetus results in substantial economic losses throughout most of the cattle-rearing areas of the world where natural breeding is relied upon.
T. foetus infection is recognized as a significant cause of bovine infertility. This infection touches upon each component of the producer's net income. Specifically, it reduces both, the total number of calves produced and their suckling-growing period. Moreover, T. foetus infection results in calves being produced later in the season, thus giving a lighter calf crop which is less uniform in age and weight. Under these circumstances, the calf crop would be unlikely to realize its full market value. In addition, increased feed, treatment, culling and stock replacement costs are incurred. All tolled, it is estimated that, in 1992, T. foetus infections will cost the U.S. beef industry approximately 500 million U.S. dollars.
Transmission of T. foetus is almost exclusively by coitus. Infection of this type is confined to the reproductive tract with the preferred sites of infection in the bull being the prbputial cavity and the urethral orifice. The prevalence of T. foetus infection in bulls generally increases with age and is attributed to the deepening of crypts in the preputial epithelium of older bulls.
Various topical treatments have been proposed for the treatment of T. foetus infection in bulls. Examples of such treatments include the implementation of acriflavine salves, chlorohexidine and metronidazole. For the most part, these methods of treatments have proven to be only marginally effective.
The therapeutic value of several 5-nitroimidazole compounds has been established in the treatment of T. foetus infection. While multiple oral doses of dimetridazole have been documented as being moderately effective, they can result in undesired side effects such as rumen stasis and anorexia. A single intravenous administration of dimetridazole has also been documented as being moderately effective. However, this too can cause undesired side effects such as respiratory difficulty, ataxia and lung collapse.
Although treatment of T. foetus infection with 5-nitroimidazole compounds may be moderately effective, it is prohibitively expensive. For example, in 1991, the cost of each oral administration was approximately $100.00 (U.S.) per bull; and, the cost of an intramuscular administration was approximately $50.00 (U.S.) per bull. Moreover, the use of 5-nitroimidazole compounds is not approved by the U.S. Food & Drug Administration. One such compound, ipronidazole has been declared a suspect carcinogen.
As can be seen, the control of bovine trichomoniagis poses an enormous problem, even in the comparatively well-managed beef herds of the United States. This problem has grown to such proportions that a joint task force comprising the Californian Cattlemen's Association, the U. S. Animal Health Association and the Livestock Disease Research Laboratory has declared bovine trichomoniasis research as having the highest priority.
Other examples of free-living and parasitic protozoans which infect humans or animals include, without limitation, Trichomonas vaginalis, Tritrichomonas gallinae, Gtardia lamblia, Cryptosporidium parvum, Cryptosporidium sp., and Plasmodium Sp. These and other protozoan organisms are known to cause the following diseases: trichomoniasis, giardiasis, cryptosporidiosis, malaria, babesiosis, theileriosis, toxoplasmosis and leishmaniasis, in addition to all other intestinal and systemic protozoan infections.
An effective treatment against free-living and parasitic protozoan infections of humans and animals is needed. Such a treatment must not only lack the harmful side effects of metronidazole, but also, must be more effective than bacitracin.